1. Field of the Invention
The present invention relates to a laminated steel sheet used for welded cans such as food cans and, more particularly, a laminated steel sheet for cans which is easily welded and has a film structure for preventing a decrease in adhesion a of laminated resin film in the can making and filling processes.
2. Description of Related Art
There are several methods of manufacturing cans: the soldered can making method, welded can making method, cementing can making method, draw and ironing can making method, draw and redraw can making method, etc. The welded can making method uses simpler equipment than any other methods and can be carried out by easy and reliable operation. The cans made by this method have welded portions with high strength, and withstand severe working. For these reasons, the welded can making method has come into wide use rapidly.
In the soldered can making or welded can making methods, to make a round can, for example, opposed ends of a sheet cut into a square necessary for the can body are joined, and the joint is welded by soldering or electric resistance welding. For plating of steel sheet for cans for the soldering or welding can making methods, tin was initially used.. In the welded can making method, however, far less deposit of tin is required than the soldering can making method, or the deposit of tin is not required at all, so that thin plating or combined use of other metal is carried out. From this point of view, the rationalization of the welded can making method is anticipated.
The body and ends of food cans are painted. The painting includes thick-film painting for enhancing corrosion resistance of the inner surface of the can, painting for protecting the outer surface, printing, and the like. When painting or printing is performed, a plated steel sheet is cut into a predetermined size, and then each piece of sheet is painted, dried, or printed. This process requires a high transportation cost as well as measures against environmental pollution due to painting. Therefore, these disadvantages have been some of the problems to be solved in can making.
As the first stage for solving the above problem, what is called the precoating, in which a plated steel sheet is painted in advance or a resin film is laminated, was tried. However, the merely precoated steel sheet had a problem in that the resin film in the welded area must be removed in welding.
So far, a steel strip on which a film is laminated in a band form except for a welded area has been proposed for welded cans. For instance, Japanese Patent Laid-Open H-3-236954 discloses a material for cans on which a thermal plasticity film is laminated over a width corresponding to the height of a can except for a non-laminated area 2 to 5 mm wide. The disclosed resins for thermal plasticity film are polyester, polypropylene, polyethylene, and nylon.
In the steel sheet for cans proposed above, the decrease in adhesion strength between coating film and base due to the thermal effect in welding, touch-up painting of welded area, or heating is not considered. As a result, there is a problem of insufficient adhesion after retorting, i.e., heating.
The present invention was made to solve this problem. It provides a laminated steel sheet for welded cans which ensures high adhesion strength even if heat is applied and has high adhesion after retorting by forming a film structure for overcoming the causes for decreasing adhesion strength in the can making process.